>Anyone have an idea on how I can measure capacitance?
>I wanted to charge a cap with a constant current and measure
>the time it takes to reach Vref. I have some _serious_ doubts
>about this though.
>
>How do commercial meters work then?

Well what I would do is use the good old 'first principles' formula for
capacitance reactance. This will tell you the impeadence of the capacitor.
You could then include your cap in part of a bridge network and accurately
find the capacitance that way.

The formula is... Z(The Impeadance)= 1/(2*pi*F*C) where F is your
applied frequency, C is the farads of your capacitor, and pi is 3.1415927
(or close to it). So rearange the formula and you will get your value of
impeadance to put into your bridge network.

> Well what I would do is use the good old 'first principles' formula for
> capacitance reactance. This will tell you the impeadence of the capacitor.
> You could then include your cap in part of a bridge network and accurately
> find the capacitance that way.
>
> The formula is... Z(The Impeadance)= 1/(2*pi*F*C) where F is your
> applied frequency, C is the farads of your capacitor, and pi is 3.1415927
> (or close to it). So rearange the formula and you will get your value of
> impeadance to put into your bridge network.
>
Since Z is dependent on frequency, you would like to apply a single
frequency to the bridge, ie. a sinusoid. You would also need a pricision
rectifier to measure the amplitude of the voltage across the bridge.

Another approach could be to use the capacitor in an RC oscillator and
measure the frequency. You would need to know R quite accurately as well.

You could also charge the capacitor with a constant, known current and
measure the time it takes for the voltage to reach a known threshold.
The current is in Coulomb per Second (C/s). The voltage across a
capacitor is given by V=Q/C where Q is the charge in the cap and C is the
capacitance in Farads. So, if the capacitor charges up from zero charge,
the capacitance is given by:
C = Q/V
C = I*t/V
where I is the constant current and t is the time it took for the
voltage to reach V.
Well, thats the theory. Doing the maths on an 8 microprocessor, whithout
multiply or divide can become tricky. Appropriate scaling might help a
bit.

----------
> From: N STEENKAMP [M.ING E&E]
> > Well what I would do is use the good old 'first principles' formula for
> > capacitance reactance. This will tell you the impeadence of the
capacitor.
> > You could then include your cap in part of a bridge network and
accurately
> > find the capacitance that way.

What about rude and crude time constant approach like AN512 describes.
Ping the capacitor to charge it and then measure the time it takes for the
capacitor to discharge to below the ports turn-on threshold. But, here you
are measuring capacitance instead of resistance.

Problem is that it would have a pretty narrow and presumably non linear
range. Might be OK for Go/NoGo testing though.

>You could also charge the capacitor with a constant, known current and
>measure the time it takes for the voltage to reach a known threshold.
>The current is in Coulomb per Second (C/s). The voltage across a
>capacitor is given by V=Q/C where Q is the charge in the cap and C is the
>capacitance in Farads. So, if the capacitor charges up from zero charge,
>the capacitance is given by:
> C = Q/V
> C = I*t/V
> where I is the constant current and t is the time it took for the
>voltage to reach V.
>Well, thats the theory. Doing the maths on an 8 microprocessor, whithout
>multiply or divide can become tricky. Appropriate scaling might help a
>bit.
>
>Hope it's useful
>Niki

The problem with constant current charging is that you need a programmable
current source. Partly to cover a nine decade range of capacitors, and also
to allow for leakage currents by taking measurements at two different
charging rates. This isn't too straightforward so a.c. measurement of some
sort is favourite.

My Metrix MX56 DVM does just that. The currents used are switched by the
autoranging, being 100nA, 1uA ... 1mA. The capacitor is discharged after
each measurement cycle, presumably through a simple resistance
(exponential decay of cap voltage).

Seems to work, it's certainly well within the claimed 1% accuracy over
the 50nF to 50mF FSD ranges, although I suspect that there is a bit of
tweaking done in the firmware - the charging ramp does not look
perfectly linear on the scope :)

Otherwise you could play with synchronous rectifiers to measure just the
imaginary component of the capacitor impedance. I've just finished a
project to do the opposite - measure only the resistive component of the
impedance that way. Works quite well.

Martin Latecka wrote:
>Anyone have an idea on how I can measure capacitance?
>I wanted to charge a cap with a constant current and measure
>the time it takes to reach Vref. I have some _serious_ doubts
>about this though.
Your principle is good for any reasonabable size capacitors ( 100pF - 10000 uF)
Keep the curent generator constant at normal measuring tempereatures.
If you want more precision, add also low reference and measure dT/dV

>How do commercial meters work then?

My company manufactures high power capacitor meters and they put voltage to
capacitor and measure current, so it basically is impedance meter.
C= 2*pi*f*i/V.
The speciality of my meter is: it can find one faulty capacitor from many
parallel capacitors. By the way big capacitor bank is 300 meters long and
they have 10000 pcs 70 kg capacitors in 500kV potential.

>Anyone have an idea on how I can measure capacitance?
>I wanted to charge a cap with a constant current and measure
>the time it takes to reach Vref. I have some _serious_ doubts
>about this though.

Why bother about constant current? You are measuring time constant,
which is expressed adequately AND LINEARLY in terms of the charging
resistance, the capacitance and the (logarithm of the) ratio of
initial to terminal voltage. If therefore you measure the time it
takes for the voltage to decay to a fixed proportion (resistor divider;
comparator) of the charging voltage (regulated and stable, but not
precise) you only require precision of the three resistors and timing
reference, and short-term stability of the charging voltage.

The only design concern is how you implement ranging, as you need to
minimise stray resistance and capacitance, the former when measuring
large capacitances, and the latter, small. If you are only concerned
with a small range, this is no problem. A version of manual ranging
selection is to built alternate probes for different ranges.

I have considered (but not built) a probe for picofarad and
fractional-picofarad measurements only, to be built using SMD into the
probe itself. The circuit and components for values upward of 100uF
into the multiple mF (millifarad) range would be quite different, such
as VFETs for switching instead of CMOS.